How to get exactly height of UILabel with line space?

I'm writing a expandable tableView header, I need to set the height of header accurately in tableView delegate method. I'm now using the following method to calculate the height of my header(a multi-line label):

However, I've found that the height calculated out is just about the text without include line space.
So how can I get the line space height of the label? or how to get accurate height of UILabel with line space?